Method and means for the transmission of low-frequency calls intelecommunication systems by interrupting the carrier



Apnl 3, 1951 M. A. LALANDE I METHOD AND MEANS FOR THE TRANSMISSION OF LOW FREQUENCY CALLS IN TELECOMMUNICATION SYSTEMS BY INTERRUPTING THE CARRIER 2 Sheets-Sheet 1 Filed July 25, 1947 WWW INVENTOR MARC A. LALANDE BY K ATTORNEY METHOD AND MEA NS FOR THE TRANSMISSION OF LOW FREQUENCY CALLS IN TELECOMMUNICATION SYSTEMS BY INTERRUPTING THE CARRIER 2 Sheets-Sheet 2 A1333, 1951 M A LALANDE 2,547,020

Filed July 25, 1947 j ij INVENTOR MARC A. LALANDE ATTORNEY Patented Apr. 3, 1951 UNITED STATES METHOD AND MEANS FOR THE TRANSMIS- SION OF LOVV-FREQUENCY CALLS IN TELECOMMUNICATION SYSTEMS BY IN- TERRUPTING THE CARRIER Marc Andi- Lalande, Paris, France, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application July 25, 1947, Serial No. 763,618 In France December 26, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires December 26, 1964 9 Claims- The present invention relates to processes ano. means for the transmission low frequency calls in telecommunication systems such as carrier current systems, radio links and/or voice frequency telephone systems.

According to some of its features, the invention provides a low frequency call transmission process in which, at the outgoing end, a modification of the carrier current is effected at a relatively low frequency by means that introduce important harmonics and insure a great depth of modulation, and at the receiving end there takes place a selection of one or more harmonics located in the portion of the range of voice frequencies in which the voice components still have only low or artificially reduced amplitudes.

The invention will be explained in detail in the following description, given with reference to the appended drawings, in which:

Figs. 1 and 3 respectively illustrate the transmitting and receiving portions of circuits of a radio system that employ features of the invention for the transmission and reception of call signals.

Fig. 2 illustrates the nature of a high frequency train of impulses employed for the transmission of calls.

Figs. 4 and 5 illustrate two modifications of the circuits of Fig. 1 for the case in which the invention is applied to voice frequency telephone systems.

Figs. 6 and '7 illustrate variants of portions of the circuit of Fig. 1 for the transmission of calls, and

Fig. 8 illustrates the use of a ringing relay, but

mounted according to a method that provides a slow cadence, as a variant of the devices of Figs. 1 and 4 to 7.

Referring to Fig. 1, the very low frequency call (e. g. at C. P. S.) arrives over line I, passes through the windings of transformer 2, and at the terminals of condenser 3 finds a path across rectifier 4 to relay 5 for the alternances of one direction, and across rectifier 6 and resistance '1 for those of the opposite direction.

At its rest contact, relay 5 closes the plate feed circuit of the oscillator tube 8. Under the action of the 20 C. P. S. call current, relay 5 opens the plate feed circuit 20 times a second, and this causes the sending of a high frequency train of impulses with substantially rectangular envelope, like that shown in Fig. 2.

If the device is to be used in a transmisssion system of the suppressed carrier current, it is necessary to add to this circuit a device that Ill) will cause emission of the carrier current during the duration of the call. Since such a device is well known in the art, it will not be described in detail.

Fig. 3 illustrates the members affected by the transmission of calls at the receiving end. The high frequency signal received by the antenna 9 traverses a certain number of high frequency members [9 before terminating at the detector stage II which restores a low frequency signal. The subsequent members [2 are as a rule provided for the transmission of telephone frequencies and cannot permit passage of a frequency of the order of 20 C. P. S. However, the substantially rectangular envelope of the high frequency signal implies the existence of harmonies of 2c C. P. S. having amplitudes that are not negligible. The harmonics of the order of C. P. S. can be transmitted by the telephone frequency circuits and reach the transformer l3.

The condenser I4 inserted in the primary windin of transformer I3 permits passage of the telephone frequencies which, by transformation, reach the line 25 across the rest contacts of the relay of line 24. Furthermore condenser l 4, selfinductance I 5 and condenser l6 form a circuit tuned to about 120 C. P. S.; a voltage of 120 C. P. S. develops at the condensers terminals, while the other frequencies are substantially eliminated. Since the voice frequencies of the order of 120 C. P. S. are of very low amplitude with respect to that of the 20 C. P. S. call harmonies, the voltage developed by the voice at the terminals of condenser It will be negligible in practice.

The voltage developed by the call charges condenser I'l, makes the grid of tube l8 strongly negative and causes release of relay is located in the tubes plate circuit. The resistance of grid 2& is provided with a high value in order to give a time constant of the order of 0.25 second to the condenser |'l-resistance 2Q circuit for the double purpose of keeping relay !9 on its rest contact during the duration of the train of impulses and of retarding its return into operation in order to prolong the duration of the call in case the same should be abnormally short, and thus give relays '21, 22 and 24 time to opcrate.

By means of its rest contact, relay [9 closes the circuit of relays 2| and 22 which control each other and are adjusted to beat at about 20 C. P. S. Furthermore, relay 2| periodically reverses the direction of the current in transformer 23, producing in the secondary an alternating current cf 20 C. P. S. At the same time, relay 2 is set in operation by the rest contact of relay i9 and it sends into line 25 the call current proceeding from transformer 23.

It can be seen that there is thus created a call. transmission system for radio links for carrier currents on cables that does not require having recourse to the conventitonal methods of frequency transposition.

The call transmission process and means that have just been described can be advantageously. applied to ordinary telephone systems, in which.

case the continuous current takes the part of the carrier current. It is then possible, for example, to periodically interrupt the plate feed circuit of an amplifier tube or, if there is none in the circuit, to use an. arrangement like that shown in Fig. 4 or Fig. 5, in which the corresponding elements of Fig. 1 are indicated by the same reference numbers. The telephone line is in this case directly connected to the terminals of the secondary winding of transformer it.

The contact of relay v closes periodically, at the cadence of the call, the circuit of a battery on line 25 (Fig. i) or on an add-itionalwinding Zi of transformer 2 (Fig. 5). 1 v

In this case of application, the receiving circuit is reduced to the elements it to 25 of Fig. 3.

In the case in which the described system is applied to bilateral connections over cable, the transformers 2 (Fig. 1) and is (Fig. 3) may be the ones existing in the 4-wire terminals connected by the long distance cable.

Instead of completely cutting ofi the voltage of oscillator tube El, it is possible to merely provide for its modification, e. g. as shown in the schematic diagram of Fig. 6, in which the contact of relay 5 short-circuits a resistance 29.

In order to favor the appearance in call signals of harmonics of the'desired order, use may also' be made of an arrangement like that shown in .Fig. 7, in which a circuit 3i), tuned to a frequency of between 160 and 150 C; P. S. for the casein question-of a call at 20 C. P. S., is inserted in the plate circuit andis short-circuited by the contact of relay 5.

Rectifiers 4 and 5 shown in the various drawings may be eliminated by utilizing a relay 5that is capable of following the positive and negative alternances of the call current; in particular, use

may be made of a slow ringing relay like that shown in Fig. 8.

Any Well known types of circuits for'protection of the contacts or of shape correctors of the periodic currents may also be inserted in the circuits in question.

At the receiving end, the 20 C. P. S. current generating assembly formed by relays 2|, 2'2 and transformer 23 may be eliminated if a source of current of suitable frequency is available at that place. Self-inductance i5 and'condenser it of the harmonics selecting circuit may of course be reversed, since the 120 C. P. S. voltages at the terminals of the self-inductance and of the condenser are of the same order of lmagnitude.

Various further modifications and adaptations may be made in the described and illustrated arrangements without departing from the scope of the invention.

I claim:

1. In. a signal communication system utilizing a carrier current for transmission, the method of transmitting and reproducing calls of a signal frequency low with respect to thefrequencies in a predetermined signal range, comprising the steps of modulating said carrier current at the transmitter with a frequency corresponding to said low frequency call signals to produce one or deep level whereby a signal containing harmonics of the call frequency contained within said predetermined signal range is produced and the reproducing step includes generating call signals in response to harmonic energy of call signal frequency independently of frequency conversion.

3. In a signal communication system having means for transmitting low frequency call signals on a carrier current, the combination comprising meansfor-modulating said carrier current with a modulation call signal of a frequency below a predetermined signal range, means whereby said last named means is operative to produce a carrier wave including harmonics of said modulation signal within said predetermined signal range, means for receiving said last named carrier wave including means for selecting at least one harmonic of said modulation signal located in said predetermined signal range and means controlled by said selected harmonics for reproducing said low frequency call.

- i. The signal communication system as set forth in claim 3 wherein said transmitting means includes means for generating a carrier current and means for controlling said generating means, and wherein said modulating means includes a relay device responsive to low frequency call signals connected to said control means, means whereby said relay device is operative to control the flow of carrier current and said receiving means includes, a relay device responsive to said selected harmonics within said predetermined signal range,- a call frequency signal generator controlled by said relay device and means whereby said call frequency generator is operative to reproduce said call frequency independently of frequency conversion.

*5. In a low-frequency call transmitter the combination comprising means for generating and transmitting a carrier current of a predetermined frequency, means for modulating said carrier current with a first signal within a predetermined signal range, a relay, means for rendering said relay selectively responsive to call signals of a frequency below said signal range and means under control of said relay being operative to modulate said carrier current at a rate proportional to said call signal frequency to provide a modified carrier waveform having harmonics of said call frequency contained within said predetermined signal range.

6. A transmitter in accordance with claim 5 wherein said modulating means controlled by said relay includes a high frequency electronic oscillator, said relay being operatively connected to control the plate current of said oscillator.

7. A low frequency call transmitter inaccordance with claim 5, including an electron discharge device controlling the carrier current and having a plate controlled circuit including a resistance and a shunting control therefor under control of said relay.

8. A low frequency call transmitter in accordance with claim 5, including an electron discharge device having a plate controlled circuit includ- UNITED STATES PATENTS ing a parallel resonant circuit approximately Number Name Date tuned to a desired harmonic and means for in- 1 624 506 Pruden 12 1927 terrupting said plate control circuit under con- 1:862;608 June 1932 trol of Sand relay- 5 2,162,268 Meszar June 13, 1939 9. A low frequency call receivlng and reproduc- 2 244 240 Blumlein June 3 1941 ing system comprising a rece v r operative to 2 0 5 c u 1 Way 26 1942 receive a carrier current modulated in accordan e 2299945 Wendt Oct 042 with a low frequency call signal, means at i 2306687 Cox 1942- receiver for generating signals at th call Signal 10 2319306 ggfl H May 18 1943 frequency and means solely responsive to h 2442238 HaringS May 5 43 monics of said call frequency signal as contained fl in said modulator carrier current for controlling OTHER REFERENCES said signal generator to reproduce said low fre- RCA Review, Mal-ch 194g 1 1X, 1, 1 quency call signals. 1.5 vision D. C. Component, by K. R. Wendt, pp.

MARC ANDRE LALANDE. 85-111.

REFERENCES CITED The following references are of record in the file of this patent: 

